Mass Flow Controllers

In plasma-etch, chemical vapor deposition and many other processes, accurate metering of gas flow into the process chamber is critical because, beyond the process wafer, all materials that participate in the etch or deposition are introduced in gas form. In a majority of these processes, two or more of these gases react to produce the essential film or passivation layer and even slight deviations in gas flow—even on the order of 1%— can cause the process to fail.

A mass flow controller (MFC) is a device used to measure and control the flow gas into the process chamber. A gas mass flow controller is designed and calibrated to control a specific type of gas at a particular range of flow rates. The MFC can be given a setpoint from 0 to 100% of its full scale range but is typically operated in the 10 to 90% of full scale where the best accuracy is achieved. The device will then control the rate of flow to the given setpoint. MFCs can be either analog or digital.

All mass flow controllers have an inlet port, an outlet port, a mass flow sensor and a proportional control valve. The MFC is fitted with a closed loop control system which is given an input signal by the operator (or an external circuit/computer) that it compares to the value from the mass flow sensor and adjusts the proportional valve accordingly to achieve the required flow. The flow rate is specified as a percentage of its calibrated full scale flow and is supplied to the MFC as a voltage signal.

While numerous technologies have been developed to accomplish gas flow metering, the semiconductor market has focused largely on two: the thermal- based mass flow controller (MFC) and the more recently introduced pressure-based flow controller.

Today, 1% accuracy is required for challenging applications and Pewsey believes we will soon see a requirement for 0.5% accuracy. Tighter flow repeatability is also required for chamber matching.

New MFC designs feature real-time rate-of-decay flow error detection technology to continually test for changes in the device’s performance. Data can be used to improve accuracy at critical low-flow set points, set up alarm limits for critical performance parameters and monitor trends for predictive maintenance.

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